Carbonization method for cellulosic fibers



United States Patent 3,479,150 CARBONIZATION METHOD FOR CELLULOSICFIBERS Carlos L. Gutzeit, Long Beach, Calif., assignor to Hitco,Gardena, Calif. No Drawing. Filed July 14, 1965, Ser. No. 472,043 Int.Cl. C01b 31/07 US. Cl. 23209.1 25 Claims ABSTRACT OF THE DISCLOSUREFibrous cellulosic materials are carbonized in the presence of an acidicoxidizing gas at a temperature between I about 300 F. and below atemperature which results in substantial impairment of the integrity ofthe fibrous materials until the exothermic reaction between theoxidizing gas and materials is substantially completed. The resultingmaterial is thereafter heated at a higher temperature in the essentialabsence of oxygen and preferably in the presence of halogen or analkaline reducing gas until carbonization is substantially completed.

The present invention generally relates to carbonization and moreparticularly relates to an improved method of carbonizing fibrouscellulosic materials.

Amorphous and graphitic carbon products are becoming increasinglyimportant in various applications, for example in the space industry,where thermally insulative, ablation resistant materials are needed.Carbon products heretofore have been prepared by a wide variety oftechniques in a number of forms, including monolithic forms and fibrousforms. The fibrous forms are newer and present special productionproblems. They include amorphous carbon and crystalline carbon(graphite) textiles, rovings, extended filaments, cord, tape and thelike.

Whereas strong monolithic forms can be'fabricated by special rapidbinding and hot pressing techniques, fibrous forms do not lendthemselves to such fabrication techniques. Instead, time consumingcarbonization procedures must be utilized to preserve, as much aspossible, the strength and flexibility of the fibrous startingmaterials. However, present commercial techniques still result insubstantial'decreases in strength, flexibility and durability of thefibers. In many instances, fiber integrity is seriously endangered.Moreover, commercial fibrous carbon products usually vary considerablyfrom lot to lot in the indicated characteristics. In addition,considerable difficulties have been encountered in producing carbonizedfibrous materials in a uniformly high yield. Commercial methods usuallyresult in a recovery of only about 18-20 wt. percent of the product asopposed to a theoretical yield of about 44.4 wt. percent of carbon inthe case of rayon and the like.

Inasmuch as the usual commercial carbonizing method ordinarily isrelatively complicated, time consuming (of the order of 5-10 days) andexpensive, it is obvious that it is of considerable importance that ashigh a yield as possible of uniformly high quality product be obtained.A further difliculty has been encountered in that existing commercialmethods are not well adapted to vary selected characteristics of thecarbon products. Thus, certain applications require fibrous carbonproducts of reduced surface area. Other applications desire high surfacearea products. Such products have been difficult or impossible toachieve.

It would be very desirable to provide an improved method ofcarbonization of fibrous cellulosic materials, which method should becapable of resulting in improved yields of uniformly high quality (highstrength, flexible and durable) carbon products whether amorphous, orcrystal- 3,479,150 Patented Nov. 18, 1969 line or any mixture thereof.It will be understood that, unless otherwise specified, reference hereinto fibrous carbon products is intended to encompass fibrous amorphouscarbon products and fibrous carbon products which contain up to aboutpercent graphite.

Accordingly, it is a principal object of the present invention toprovide an improved carbonization method.

It is also an object of the present invention to provide a simplerelatively rapid and inexpensive method of carbonizing fibrouscellulosic materials to fibrous carbon products in high yield.

It is further object of the present invention to provide an improvedmethod of carbonizing fibrous cellulose materials to fibrous carbonproducts of controlled characteristics, for example, relatively hightensile strength, flexibility, and durability, and controlled surfacearea and porosity.

It is a still further object of the present invention to provide animproved carbonization method capable of producing fibrous carbonproducts of uniformly high quality.

These and other objects are accomplished in accordance with the presentinvention by providing an improved carbonization method. The methodcomprises partial carbonization of fibrous cellulosic materials atrelatively low temperatures in the presence of selected acidic oxidizinggas. The method is believed to operate by effecting rapid and controlleddehydration of the fibrous cellulosic material, thus initiatingcontrolled breakdown of the cellulosic material so as to utimatelyresult in a substantial improvement in yield and strength, flexibilityand durability of the final fibrous carbon product. Such dehydration canbe accomplished substantially completely and without substantialdeterioration of the fiber integrity in the final product. Moreover, itcan be accomplished at a lower temperature than possible in conventionalcarbonization techniques.

The treated product of the described step of the present method can thenbe subjected to relatively rapid controlled higher temperatureessentially complete carbonizing and, if desired, subsequentgraphitization to provide the final desired product.

In one embodiment of the present method, a selected alkaline reducinggas contacts the acidic oxidizing gastreated product during such highertemperature carbonizing. This contacting has the unexpected result ofsubstantially decreasing the total surface area and porosity of theultimate fibrous carbon product.

In another embodiment, contacting with selected acidic oxidizing gas iscontinued throughout higher temperature carbonizing to assure a veryhigh purity product.

Accordingly, the present method is relatively simple, inexpensive andrapid and provides a final fibrous carbon product of uniformly highquality, including high tensile strength, flexibility and durability,and of controlled surface area. Of considerable importance, the productis obtained rapidly and in an improved yield. The method is adapted forbatch, semi-continuous or continuous operation. Further advantages ofthe present invention will be apparent from a study of the followingdetailed description.

Now referring more particularly to the present method, an initially lowtemperature heat treatment of fibrous cellulosic material in thepresence of a selected acidic oxidizing gas is carried out in order toeffectively initiate carbonization. It is believed that the acidicoxidizing gas, dehydrates the fibrous cellulosic material and speeds thecarbonization process. The acidic oxidizing gas-contacting stepconstitutes the first major step of the method.

The fibrous cellulosic material may be any fibrous cellulosic material,natural or artificial, e.g., reconstituted,

for example, cotton, rayon, or the like, in woven or unwoven fibrousform. Thus, the starting material can be in carded or uncarded form,chopped fiber form, felted, etc. Woven textiles are used extensivelybecause of their inherently great utility. It is preferred that thestarting material be substantially devoid of foreign matter, includingfinishes, coatings, impregnants and the like. Accordingly, variousconventional washing operations, solvent treatment steps and the likecan be performed to bring the starting material to the desired state ofpurity. Such steps are not required, however, so long as the startingmaterial is substantially free of foreign matter which would interferewith the desired carbonization and/ or reduce the desired purity of thefinal product.

, As an optional, although preferred, preliminary step, prior tocontacting the starting material with the acidic oxidizing gas, thestarting material can be heated in an inert gas, for example, nitrogen(of suflicient purity), helium, argon or the like, to any suitabletemperature up to about 350 F. to remove surface moisture from thestarting material. Temperatures above about 350 F. are not recommended,because of difficulties in preventing partial degradation of thecellulosic material. Such preliminary heating can occur over, forexample, -15 minutes or the like, preferably while sweeping the surfaceof the fibrous cellulosic material with the inert gas. Such treatmentcan be terminated when moisture is no longer detected in the exitinginert gas. Such treatment in some instances slightly increases theultimate yield of fibrous carbon product.

After the described optional moisture removal step, the fibrouscellulosic material is subjected to the required low temperaturetreatment (first major step) in the presence of the acidic oxidizinggas. The temperature at which the acid oxidizing gas treatment iscarried out is one which is sufiicient to facilitate the relativelyrapid dehydration of the fibrous cellulosic material in the presence ofthe acidic oxidizing gas. For most purposes, temperatures below about300 F. are too low to permit a reaction rate sufiiciently rapid andcomplete for commercial preparation of the carbon fiber products.Temperatures within the range of from about 300 F. to above 350 F. arepreferred. It is within this range that the desired reaction occursrelatively rapidly but at a readily controllable rate. Temperatureshigher than about 350 F. can be used, for example, up to about 450 F. orso. However, no advantage is obtained by using the same. Moreover,difficulties may occur in controlling the rate of the reaction so as toprevent undesired changes in the fiber integrity or structure of thematerial under treatment. Accordingly, for most purposes, the reactionis carried out at temperatures of between about 300 F. and about 350 F.It will be noted that such temperature is substantially belowconventional carbonization temperatures.

The selected acidic oxidizing gas is any suitable acidic oxidizing gaswhich does not leave an appreciable residue in the carbonized product.For most purposes halogens, namely chlorine, bromine and iodine arepreferred. However, halogen-yielding mixtures of hydrogen halides andoxygen (e.g. air) can be used as substitutes for the halogens, as by amechanism believed to be analogous to the well-known Deacon process forchlorine generation. The oxygen concentration of such a mixture shouldbe no greater than that necessary to convert all of the hydrogen of suchhalide to water i.e. no more than a stoichiometric amount. However, anabout stoichiometric amount of oxygen should be used so that thehalogen-yielding reaction is carried to substantial completion. Suitablemixtures of the described acidic oxidizing agents can be provided. Itwill be noted that uncombined oxygen, as for example, in air cannot besuccessfully used by itself, in the absence of the acidic gasesabove-described. Oxygen is not sufficiently reactive for the describeddehydration at the indicated temperatures. Moreover, oxygen in anysubstantial concentration presents a certain hazard in that combustionof the fibrous cellulosic material can occur if the temperature of thematerial is inadvertently allowed to rise to above about 490 F.Accordingly, for the purposes of the present invention, the oxidizingagent is limited to acidic oxidizing gas, as described. Preferably, theacidic oxidizing gas is diluted with inert gas, e.g., pure nitrogen,helium, argon, etc. or mixtures thereof, to a suitable concentration,usually between about 1 and about 10-15 vol. percent, depending upon thereactivity of the acidic oxidizing gas, the reaction temperatureselected, the surface area of the material, and other factors. Othersuitable concentrations of the acidic oxidizing gas can be utilized.

The exothermic reaction which takes place when the acidic oxidizing gasis exposed to the fibrous cellulosic material at the initial reactiontemperature within the above-described range, should be sufficientlycontrolled so as not to unduly stress the fibrous cellulosic materialand so as to avoid preparation of a brittle product. Thus, it isgenerally desirable, when carrying out such contacting, to firststabilize the temperature of the fibrous cellulosic material within thedescribed reaction temperature range in an inert gas containing noappreciable concentration of the acidic oxidizing gas. Such acidicoxidizing gas can then be added in controlled concentration insufiicientto cause the exothermic reaction to proceed so rapidly as to raise thetemperature of the cellulosic material to more than the desired maximumtemperature, e.g., about 350 F.

Accordingly, it is preferred to initially establish a temperature ofabout 300 F. in the cellulosic material in the contact zone in an acidicoxidizing gas-free inert gas environment, and then bleed the acidicoxidizing gas into the contact zone at a rate which assures no more thanabout a 50 F. rise in temperature in the cellulosic material during suchtreatment. Normally, the concentration of the acidic oxidizing gas isincreased from a few vol. percent to an ultimate concentration withinthe desired range. Such acidic oxidizing gas is consumed during thereaction and must be replenished until the exothermic reaction iscomplete, usually within about 5-10 minutes, although longer and shorterperiods of time are contemtemplated.

When the desired dehydration of the cellulosic material is completed atthe reaction temperature, further utilization of the acidic oxidizinggas substantially stops. Further addition of acidic oxidizing gas to thecontact zone, or further maintenance of the contacting thereof with thecellulosic material has no further effect on ultimate yield of thecarbon product. The exothermic reaction can be carried out with orwithout sweeping of the contact zone with the acidic oxidizing gas.However, sweeping is preferred.

It will be understood that, if desired, the cellulosic material can beheated to reaction temperature in the presence of inert gas whichalready contains a controlled concentration of the acidic oxidizing gas.However, adequate temperature control of the cellulosic material in thecontact zone is more easily achieved by following the previouslydescribed or a similar procedure.

Although the exact mechanism of the reaction occurring in the describedlow temperature acidic oxidizing gascontacting step (first major step)is not fully understood, it is believed to involve the describeddehydration of the cellulose, which makes easier the completecarbonization of the cellulose with a lowered loss of carbon and withretention of fiber integrity, flexibility and tensile strength. Whateverthe actual mechanism involved, the improved results are reproducible.

Once the dehydration step is carried out, further loss of volatiles fromthe cellulosic material proceeds easily and rapidly at highertemperatures until high carbon content fibrous residues which maintainthe original shape of the cellulosic starting material are obtained. Asa result of the described acidic oxidizing gas treating step, fibrouscarbon product yields of up to 27 wt. percent or more with respect tothe starting material, can be obtained, in contrast to conventionalfibrous carbon yields of about 18-20 percent.

Once the low temperature acidic oxidizing gas treat ment step has beencompleted, further volatiles are removed from the fibrous material in asecond major step by increasing the temperature of the fibrous materialover a suitable period of time and in the absence of oxygen. The secondmajor step ordinarily is required, since the product of the first majorstep usually is not in a commercially utilizable form. The environmentfor the second major step may be an inert or reducing gas, or a suitableconcentration of selected acidic oxidizing gas, defined hereinafteralone or in inert gas, or an alkaline reducing gas, which may beammonia, or an organic mono or dialkylamine such as methylamine,ethylamine, propylamine, dimethylamine, diethylamine, methylethylamineor the like, undiluted or diluted with inert gas.

In the second major step, whatever the treating gas, it is essentiallyfree of oxygen, so as to prevent combustion of the fibers. Thetemperature of the fibrous material is increased at any suitable rate inincrements or continuously from the acidic oxidizing gas treatment step(first major step) temperature previously described to a temperaturewhich assures removal of most of the remaining volatiles from the carbonproduct. In the case of an inert gas environment, this temperature levelis usually about 1100-1200 F. As an example, the temperature of thecarbon fibers can be increased from 350 F. to about 1200 F. in about oneto four hours, i.e., much more rapidly than in conventional processes.

In the event that the second major step is carried out in inert gas orthe usual reducing gas such as hydrogen gas, the contact zone is sweptfree of acidic oxidizing gas from the first major step by utilizinginert gas or reducing gas as a purge. During the heating from about300-350 F. to, for example, about 1200 F., the heat treating zone ispreferably swept with the inert gas or reducing gas.

In one embodiment of the present invention, the carbonaceous fiberproduct of the first major step is treated in the second major step byincreasing the temperature thereof to a suitable level, e.g., about1100-1200 F. while maintaining contact of the product with selectedacidic oxidizing gas. Such gas is undiluted or diluted to any suitableconcentration, e.g. 1-10 vol. percent, with inert gas. Such acidicoxidizing gas constitutes one or a mixture of halogens. It will beunderstood that the use of hydrogen halides with oxygen as acidicoxidizing gas is excluded. So also are nitrous oxide and nitrogenpentoxide. This is because the elevated temperatures of the second majorstep preclude the presence of oxygen in contact with the carbon fiber,otherwise combustion of the fiber would occur. Such higher temperaturetreatment can be carried out in the previously described manner for thesecond major step, i.e., over any suitable period of time, for example,with a sufficiently slow temperature rise per hour to assure retentionof fiber integrity, strength and flexibility and with or withoutsweeping of the gas through the contact zone.

The selected acidic oxidizing gas in the second major step acts toassure essentially complete removal of inorganic impurities from thecarbon fibers so that a very high purity product is produced.

The contacting with the selected acidic oxidizing gas in the first majorstep causes the product to have a somewhat greater total surface areathan if such first major step had been carried out only in inert gas.For example, the usual surface area encountered in carbon cloth preparedfrom rayon textile samples in accordance with a conventional processinvolving initial heat treating with inert gas at from about 450 F. toabout 1100 F., followed by firing at 1800 F. is about 50-100 m. /g.,whereas when the same type of samples are treated with acidic oxidizinggas in the first major step (followed by treatment with inert gas and/oracidic oxidizing gas in the second major step) the products usually havesurface areas of up to -500 m. g. Accordingly, the combination of acidicoxidizing gas in the first major step of the present method with acidicoxidizing gas and/ or inert gas in the second major step affords a wayof controllably increasing the surface area of carbon fiber products.

The substantial increases in yield of carbon fiber products afforded bythe first major step of the present method are retained, along with theimproved tensile strength and flexibility, whether the second major stepis carried out only in an inert or usual reducing gas, or in the acidicoxidizing gas (undiluted or diluted with inert gas) or in the describedalkaline reducing gas (undiluted or diluted with inert gas). However,the alkaline reducing gas imparts additional properties to the carbonfibers. Thus, such alkaline reducing gas reduce the surface area of thecarbon fibers to a very low level, for example, to about 10-50 m. /gm.,regardless of the surface area of the material prior to said contacting.By suitably controlling the concentration and type of alkaline reducinggas and the temperature and time of contacting in the second major step,control of the surface area of the carbon fiber product can beaccomplished. Dilution of alkaline reducing gas can be accomplished withinert gas such as that referred to in connection with the first majorstep. The time and temperature utilized in the second major stepemploying the diluted or undiluted alkaline reducing gas can be aspreviously described for the embodiments utilizing acidic oxidizing gasand/ or inert gas in the second major step or any other suitable timeand temperature. Preferably, the temperature is increased to about1200-1400 F.

The carbon fiber product of the second major step usually still retainsa very low but detectable concentration of volatiles, e.g., less than 1percent by wt. Remaining volatiles can be removed from the fibrouscarbon product by carrying out an optional, although preferred, thirdmajor step i.e. firing at above 1200 F., preferably at at least about1800 F. and more preferably at about 2000 2200 F. in an oxyegn-freeinert gas environment over a suitable period of time, for example, a fewseconds to a few minutes. Cooling of the fired carbon fiber productusually is carried out to ambient temperature in inert gas and at asufficiently slow rate to assure retention of fiber integrity. Althougha vacuum can be substituted for inert gas in the firing and coolingsteps, commercial practicality usually dictates the use of inert gas.

It will be understood that the carbon fiber products provided throughthe described firing step are basically turbostratic carbon. They caneasily be converted to high quality graphitic products by heating themin an inert atmosphere to graphitization temperature, for example, toabout 4000 F. over a suitable interval of time e.g. one hour. Thegraphitization can be carried out as an extension of, in substitutionfor or in addition to the firing step previously described. Theamorphous carbon fiber material is partially or essentially completelyconverted to crystalline carbon, that is, graphite, depending upon theparticular graphitizing conditions. Although conventionalgraphite-containing fibrous products do not largely consist of graphite,they contain graphitic properties in varying degree, depending on theirextent of graphitization. It is conventional to term those productswhich exhibit at least some graphitic properties as graphite products.It will be understood that the present invention extends to amorphouscarbon fiber products, to essentially graphite fiber products and tofiber products consisting of mixtures of amorphous carbon andcrystalline carbon.

The following examples further illustrate certain features of theinvention:

EXAMPLE I Substantially pure rayon textile free of sizing and having. adenier of 1620-1690, and an average fiber diameter of about 00004 inchisp laced in 600 gm. amount in a contact zone comprising a mufilefurnace and heated to 300 F. in helium over a period of about 20minutes. It is maintained at 300 F. for 10 minutes while being sweptwith helium at the rate of about 0.1 ftfi/min. At the end of this time,the textile is free of surface moisture. Chlorine is then bled into thehelium sweep gas entering the muffle furnace to provide an initialacidic oxidizing gas concentraton of about 1 vol. percent. The chlorinecontent in the inert sweep gas is built up to a final concentration ofabout 10 vol. percent over a 10 minute period, during which time thetemperature in the furnace increases to about 330 F due to theexothermic reaction between the rayon and chlorine. The exothermicreaction terminates at the end of the 10 minute period, after which thechlorine input to the furnace is shut off and the furnace is swept freeof chlorine through the use of helium purge gas.

The temperature of the furnace and textile is then slowly raised to 1200F. (over a three hour period) while sweeping the furnace with helium atthe rate of about 0.1 ft. min. At the end of this time, the textile israpidly heated in helium to 2000 F. and held at that temperature for 30seconds while continuing to purge the furnace with helium, after whichthe textile is cooled to ambient temperature over a 2 hour period undera helium blanket, and then removed from the furnace and tested.

The resulting textile is essentially pure (substantially less than 0.5percent impurity) high quality turbostratic carbon fiber textile with atensile strength of about 19,000 and is very flexible. It exhibitsunimpaired fiber form, and weighs about 160 grams, representing about 27wt. percent of the initial weight of the rayon, a substantial increasefrom the usual 18-20 percent wt. carbon yield of conventional processes.It has a surface area of about 300 m. gm.

Separate parallel test substituting bromine, iodine,

nitrous oxide and nitrogen pentoxide in the first major 1 step providecomparable results. Further parallel tests utilizing suflicient HCl andair of HBr and air or HI and air in the first major step to yieldcomparable concentrations of the respective halogens result in carbonyields of about 2425 wt. percent.

EXAMPLE II Rayon textile identical with that of Example I is subjectedto a substantially identical treatment, except for the substitution ofbromine for chlorine in the 300- 350 F. treating step, and except forthe addition of bromine in a 10 volume percent concentration to theinert gas in heat treating to 1100 F. The bromine is then removed fromthe furnace, argon is substituted and the textile is fired at 2100 F.for seconds. The textile after firing and cooling is comparable to thatof Example I in yield, tensile strength, flexibility and fiberintegrity. Moreover, it has an essentially nil concentration ofimpurities. In addition, its surface area is about 300 m. gm.

Separate parallel tests utilizing chlorine and iodine as acidicoxidizing gases for both major steps provide comparable results. So alsodo tests utilizing nitrous oxide, nitrogen pentoxide and hydrogen halideand air mixtures in the first major step and one or more halogens in thesecond major step.

EXAMPLE II Rayon textile identical to that of Example I is subjected tosubstantially identical treatment as in Example I, except for thesubstitution of iodine for the chlorine and nitrogen for the helium inthe 300350 F. treating step (first major step) and except for thesubstitution of pure ammonia (100 volume percent concentration) for theinert gas in heat treating (second major step) to about 1400 F. Ammoniais then swept out and helium is substituted, after which the textile isfired, as per Example I, at 1900 F. for 1 minute. It is then cooled over2 hours in helium to ambient temperature and tested. The carbon fibertextile is obtained in about 27.5 wt. percent yield, has high purity anda surface area of 20 m. gm. and exhibits tensile strength andflexibility comparable to those of the carbon fiber textiles of ExamplesI and II. So also do carbon textiles provided in parallel testssubstituting the acidic oxidizing gases chlorine, bromine, nitrous oxideand nitrogen pentoxide in the first major step. Further parallel testssubstituting methylamine, ethylamine, dimethylamine, diethylamine,methylethylamine and propylamine for the ammonia in the second majorstep provide comparable results.

EXAMPLE IV The turbostratic carbon fiber textile products of Examples I,II, III are subjected to graphitization in a furnace under a heliumblanket by heating to 4000 F. over a 1 hour period. The textiles arethen allowed to cool under the inert gas blanket to ambient temperature,after which they are tested for extent of graphitization, tensilestrength, fiber integrity and flexibility. High quality graphitetextiles exhibiting high tensile strength, fiber integrity andrelatively high flexibility are produced. The graphitizationsubstntially reduces the surface area to low levels. for example, l-10m. gm.

EXAMPLE V The tests performed in Examples I to IV, inclusive, areduplicated except for the substitution (in successive series of tests)of cotton cloth, cotton roving, cotton batt, cotton felt, rayon roving,rayon monofilaments, rayon batt and rayon felt (chopped fibers) for therayon textiles of Examples I to IV. Comparable results are obtained inevery respect.

The preceding examples clearly illustrate that the present method issimple, utilizes simple equipment, is inexpensice and is relativelyrapid. Moreover, it provides fibrous carbon products of improved andcontrolled characteristics. Such products have flexibility and tensilestrength at least equivalent to conventionally produced carbon fiberproducts and have surface areas which can be controlled so as to besubstantially larger than, comparable to, or substantially smaller thanthose of conventionally prepared carbon fiber products. Yields of thecarbon fiber products have been substantially increased, as per thepresent method, from the conventional level of about 18-20 weightpercent, based on the weight of the initial material, to 25-27 weightpercent or more.

In the present method, the heat treating of the fibers at above about350 F. up to substantially complete removal of the volatiles at about11001400 F. can be accomplished in a very short period of time. Whereasconventional procedures usually require days of treatment Within thistemperature range, this step in the present method can be carried outin, for example, about one to about four hours. Firing operation canalso be relatively rapid, as can be the graphitization, if any. The lowtemperature heat treatment at about 300350 F. also is very rapid, inthat it usually can be carried out within about 5-15 minutes.Accordingly, carbon fiber products of amorphous, graphitic or mixedcarbon form can be prepared by the present method, either batchwise,semicontinuously or continuously from fibrous cellulosic material withinas little total treating time as about 2 hours. This is in contrast toconventional methods which require days to perform, for example, about8-10 days.

Despite the rapidity of the present method, the integrity of the fibers,Whether in textile, chopped fiber or other form, is maintained. Thus,volatiles are easily and rapidly removed from the fibers withoutdamaging such fibers, due to the use of the acidic oxidizing gas in thelow temperature treating step. The influence of the acidic oxidizing gasis such that the subsequent higher temperature treating step or stepscan then be greatly shortened, still without danger of damaging thefibers.

The present method is flexible in that it can be carried out utilizinginert or reducing gas, or alkaline reducing gas diluted or undilutedwith inert gas or selected acidic oxidizing gas (halogen) diluted orundiluted with inert gas, all in the absence of oxygen, for heattreating at between about 350 F. and about 1100-1400 F. The alkalinereducing gas results in a controllable decrease in surface area in theproduct. The acidic oxidizing gas during this step results in a morepurified final product. Accordingly, the present method reflectsimprovements in operational speed and efficiency, quality, yield,control of product characteristics and adaptability to various startingmaterials and various forms of fibrous carbon end products. Variousother advantages are as set forth in the foregoing.

Various changes, modifications, alterations, and substiutions can bemade in the present method, its steps and parameters and in theequipment for carrying out the stpes. All such changes, modifications,alterations and substitutions as are within the scope of the appendedclaims form a part of the present invention.

What is claimed is:

1. An improved method of obtaining high yields of carbonized fiber,which method comprises maintaining cellulosic fiber in the presence ofan acidic oxidizing gas selected from the group consisting of halogens,nitrous oxide, nitrogen pentoxide and a mixture of hydrogen halide andoxygen at a low temperature of at least about 300 F. and below about 450F. until the exothermic reaction between said acidic oxidizing gas andsaid fiber is substantially completed, and thereafter in the essentialabsence of oxygen heat treating said fiber at a higher temperature inthe presence of alkaline reducing gas selected from the group consistingof ammonia and an alkylamine until carbonization is substantiallycompleted, whereby an improved yield of low surface area carbon fiber isobtained.

2. The method of claim 1 wherein said strongly acidic oxidizing gascomprises halogen.

3. The method of claim 1 wherein said acidic oxidizing gas compriseschlorine.

4. The method of claim 1 wherein said acidic oxidizing gas comprisesbromine.

5. The method of claim 1 wherein said acidic oxidizing gas comprisesiodine.

6. The method of claim 1 wherein said acidic oxidizing gas comprisesnitrous oxide.

7. The method of claim 1 wherein said acidic oxidizing gas comprisesnitrogen pentoxide.

8. The method of claim 1 wherein said acidic oxidizing gas comprises amixture of hydrogen halide and oxygen, said oxygen being present in anamount approximating and not exceeding a stoichiometric amount necessaryto convert all of said halide to halogen and water vapor.

9. The method of claim 8 wherein said hydrogen halide comprises hydrogenchloride.

10. The method of claim 8 wherein said hydrogen halide compriseshydrogen bromide.

11. The method of claim 8 wherein said hydrogen halide compriseshydrogen iodide.

12. The method of claim 1 wherein said low temperature treatment iscarried out at between about 300 F. and about 350 F. and wherein saidhigher temperature treatment is carried out by progressively increasingthe temperature of said fiber up to about 1400 F.

13. The method of claim 1 wherein said alkaline reducing gas comprisesammonia.

14. The method of claim 1 wherein said alkaline reducing gas comprises amonoalkylamine.

15. The method of claim 14 wherein said monoalkyl amine comprisesmethylamine.

16. The method of claim 14 wherein said monoalkylamine comprisesethylarnine.

17. The method of claim 14 wherein said monoalkylamine comprisespropylamine.

18. The method of claim 1 wherein said alkaline reducing gas comprisesdialkylamine.

19. The method of claim 18 wherein said dialkylamine comprisesdimethylamine.

20. The method of claim 18 wherein said dialkylamine comprisesdiethylamine.

21. The method of claim 18 wherein said dialkylamine comprisesmethylethylamine.

22. The method of claim 1 wherein the product obtained after said higherheat treating is fired in an oxygenfree inert environment at atemperature in excess of about 1800 F. for a time sufficient to removeremaining volatiles from said product.

23. The method of claim 22 wherein graphitizing of said fibers in anoxygen-free inert environment is carried out.

24. The method of claim 1 wherein the concentration of acidic oxidizinggas is controlled such that the temperature increase of the cellulosicfiber caused by the exothermic reaction does not exceed about F.

25. An improved method of carbonizing cellulosic fiber, which methodcomprises maintaining the fiber in the presence of an acidic oxidizinggas selected from the group consisting of nitrous oxide, nitrogenpentoxide, and a mixture of hydrogen halide and oxygen, at a temperatureof at least about 300 F. and below a temperature which results insubstantial impairment of the integrity of the fiber until theexothermic reaction between the acidic oxidizing gas and the cellulosicfiber is substantially completed, and thereafter in the essentialabsence of oxygen heat treating the resulting product at a highertemperature in the presence of an alkaline reducing gas selected fromthe group consisting of ammonia and an alkylamine until carbonization issubstantially completed.

References Cited UNITED STATES PATENTS 3,116,975 1/1964 Cross et al23209.4 3,179,605 4/1965 Ohsol 23209.2 X 3,294,489 12/1966 Millington etal. 23-209.1 3,305,315 2/1967 Bacon et al. 23209.1 3,313,597 4/ 196 7Cranch et al 23 209.1 X 3,333,926 8/1967 Moyer et al. 23209.1

EDWARD J. MEROS, Primary Examiner US. Cl. X.R.

